1. Field of the Invention
The present invention relates to a field sensor communication system, and, more particularly, to a field sensor communication system in which physical quantities relating to the process being carried out at each of a plurality of plants are detected and signals denoting the thus detected physical quantities are transmitted to an upper level unit.
2. Description of the Prior Art
In general, a sensor called "a field sensor" is capable of detecting physical quantities, such as pressures, temperatures and flow rates, of each of a number of plants and of converting the thus detected values into electric signals so as to transmit them to an upper level unit via a transmission line.
The above-described transmission of the electric signals are performed in a standardize manner in such a way that the field sensor transmits an analog current signal in a range of 4 to 20 mA to the transmission line and the upper level unit receives the thus transmitted analog current signal. In general, the analog signal is transmitted from the field sensor to the upper level unit in a one-way communication manner.
In recent years, field sensors including a microprocessor have been developed and put into practical use owing to the improvement in semiconductor integrated circuit technology. According to a field sensor of the above-described type, a two-way digital signal communication can be performed through the transmission line in addition to the above-described one-way analog signal communication, so that range setting, self-diagnosis and the like operations of the field sensor can be performed in a remote control manner. A device of the above-described type has been disclosed in Japanese Patent Laid-Open No. 58-48198, and another device has been disclosed in Japanese Patent Laid-Open No. 59-201535.
A specific description will be made with reference to FIG. 4 which illustrates an example of the structure of a field sensor system which must be provided with an external power source. A field sensor 1 is operated by electricity supplied from an external power source 4 so that the field sensor 1 serves as a constant current source which transmits an analog current signal which corresponds to a detected physical quantity. An upper level receiving instrument 3 receives the analog current signal (to be called "the analog signal" hereinafter), which passes through a resistor, in series, inserted into the transmission line, by detecting the potential difference across the resistor so as to use it as the indicated value of the field sensor 1. An upper level communication device 2 is connected to the transmission line at an optional position between the field sensor 1 and the upper receiving instrument 3 or the external power source 4 so that a two-way digital signal communication with the field sensor 1 is performed.
The signal transmission to the transmission line can be performed by a method in which digital signals are used for the communication by superimposing the digital signals on the analog signals so that the values of the analog signals are not influenced; a method in which the signal transmission is performed by switching the analog signals and the digital signals; and a method in which the signal communication is performed by using only the digital signals.
However, according to the above-described conventional technologies, the transmission signal is transmitted in the form of an electric current and the reception signal is received in the form of a voltage. Therefore, the level of the reception signal is enlarged in proportion to the value of the load resistor which is, connected in series in the transmission line. As a result, the usable range of the load resistor must be narrowed in order to perform an accurate communication.
Therefore, since the usable range of the load resistor is limited, it has been difficult to expand the system by, for example, adding a novel upper level receiving instrument to the transmission line.